In general, bolometer sensors are used in a variety of applications to detect infrared radiation and provide an electrical output that is a measure of the incident infrared radiation. A bolometer typically includes an absorber element for absorbing infrared radiation and a transducer element that has an electrical resistance that varies with temperature. In operation, infrared radiation incident upon the bolometer will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained.
Recent advances in technology have enabled bolometer devices to be fabricated using MEMS technology integrated into complementary metal oxide semiconductor (CMOS) processes. In previously known CMOS fabrication processes, alumina was used for the structural layer that forms the absorber because it is capable holding its shape and also because provides a good seed layer for atomic layer deposition (ALD) of platinum, which is used as the functional layer for the absorber element. However, the use of alumina as the structural support and seed layer for the absorber element has raised issues in fabrication process that can adversely affect yield. One issue raised by the use of alumina is that, because alumina is an insulator, it must be patterned to provide the openings that allow the functional layer of the absorber to make electrical contact with the underlying metal runners. Alumina is structurally very strong. As a result, the patterning of alumina, which is done by some form of etching, can be very challenging and slow.
As an example, BCl3/Cl2 gases can be used in plasma to chemically etch alumina. However, under best-case scenarios, the etch rates are as slow as ˜1 nm/min. A physical etch process, such as ion milling, can also be used to pattern alumina. For example, argon ion milling can be used to physically sputter alumina but the etch rates are even slower at ˜0.5 nm/min. Furthermore, since very small areas of the alumina need to be patterned, the etch rates can be even slower than the typical etch rates for alumina and thereby further increase the process duration.
The relatively long processing duration of alumina decreases the yield of the fabrication process. The duration of patterning process of alumina can also result in damage to other structures during the fabrication process. For example, photoresist is typically used as a mask during alumina patterning. Long durations under processes, such as ion milling, can damage the photoresist mask as well, thereby affecting the overall yield of the process. In addition, non-uniformity of the alumina etch process can result in residual alumina being left on the surface of the underlying metal runners which can prevent good electrical contact to the functional layer (e.g., platinum) of the absorber, thereby affecting the yield of the process